metamorphic
Gneiss
Gneiss (Foliated Metamorphic Rock)
Hardness: 6-7 on the Mohs scale; Color: Speckled grayish-brown to tan with darker banding; Luster: Dull to vitreous; Texture: Medium- to coarse-grained metamorphic foliation with distinct mineral banding.
- Hardness
- 6-7 on the Mohs scale
- Color
- Speckled grayish-brown to tan with darker banding
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on the Mohs scale; Color: Speckled grayish-brown to tan with darker banding; Luster: Dull to vitreous; Texture: Medium- to coarse-grained metamorphic foliation with distinct mineral banding.
Formation & geological history
Formed under high-temperature and high-pressure regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale or sandstone) deep within the Earth's crust.
Uses & applications
Commonly used as crushed stone for construction, road base, aggregate, decorative landscaping rock, and occasionally dimension stone for countertops and building facings.
Geological facts
Gneiss is one of the oldest rock types found on Earth; the Acasta Gneiss in Canada is estimated to be over 4 billion years old, making it among the oldest intact crustal material known.
Field identification & locations
Identified in the field by its distinct alternating dark and light mineral bands (gneissic banding) and grainy, interlocking crystal texture. It is extremely common in mountain belts and continental shield regions worldwide.
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